Modern satellite communication systems provide a robust and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. These satellite communication systems have emerged as a viable option to terrestrial communication systems for carrying data traffic such as Internet traffic. A typical satellite Internet system comprises subscriber terminals, a satellite, a ground station, and connectivity to the internet. Communication in such a system occurs along two links: 1) an uplink from a subscriber terminal to the satellite to the ground station to the gateway to the internet; and 2) a downlink from the internet to the gateway to the ground station to the satellite to the subscriber terminal.
The signal quality of a link (up or down) is determined by the signal to interference-plus-noise ratio (SINR) of the link. The higher the SINR, the higher the symbol rate that can be utilized and the more efficient the error correcting codes that can be used, resulting in higher throughput speeds. Generally, for a dynamic variable rate system, each combination of symbol rate and error correcting code rate pair (SYMCOD) will be associated with a SINR operating range. This SINR range is set by the bit error rate (BER) or packet error rate (PER). Specifying a BER or PER will determine at what SINR that combination pair can operate. If a link has enough excess SINR, it may try to operate at a higher SYMCOD. However, if a link has a SINR too low for the SYMCOD it operates at, high PER will result and degrade the throughput speeds and latency of that internet link.
The SINR is determined by the ratio of the signal power divided by the combination of the thermal noise power and the power level of other sources of interference. The thermal noise is a function of the temperature of the receiving hardware, and is typically flat over the frequency operating range. By contrast, other sources of interference generally are not flat and may vary from frequency to frequency. These other sources may include adjacent satellites operating at the same frequency bands, other subscriber terminals with the local satellite operating at the same or adjacent frequencies, or other system components that operate at the same or adjacent frequencies.
For inroute transmissions from a subscriber terminal to the satellite, the uplink frequency band is partitioned into subband channels known as inroute frequency channels (IFC). Generally, each IFC operates a specific symbol rate (e.g., 1.024 Msps, 2.048 Msps, 4.096 Msps, etc.) During operation, a subscriber terminal may jump from one IFC to another following an ALOHA messaging scheme. For example, a subscriber terminal may jump to an IFC operating at a higher SYMCOD if it determines there is enough of a margin to do so while operating at an acceptable PER on the new channel.
However, because each IFC operates at a different frequency, the SINR of each channel may be different. If the subscriber terminal does not know the SINR of the target IFC in advance, it only accounts for the SYMCOD of the target IFC in determining whether there is enough margin to make the jump. Where the SINR of the target IFC is lower than SINR (i.e. higher interference) of the original channel, the subscriber terminal may operate at too high a PER when it jumps to the target IFC. Conversely, a terminal may fail to consider its ability to jump to an IFC operating at a higher SINR (i.e. less interference) and higher SYMCOD.
This IFC transitioning problem manifests itself in various settings. For example, in satellite systems having a rain adaptation feature, a terminal reduces the SYMCOD whenever a rain fade is detected to account for the lower SINR caused by the attenuation in signal power. For example, a terminal operating at a 4.096 Msps IFC may drop down to a 1.024 Msps or lower IFC. As the terminal transits channels, it is vital that the SINR of the transit IFC be known to allow correct IFC allocation. Otherwise, packet loss may occur.
As another example, when a terminal operates at an initial SINR with enough SINR margin to operate at a higher SYMCOD, it will try to operate at a higher SYMCOD by switching to a different IFC. However, if the higher SYMCOD target IFC has a lower SINR (i.e. higher interference level), the SINR margin may be insufficient to operate at the higher SYMCOD. When the terminal switches to the target IFC and sees the high packet loss and low SINR, the terminal switches to a lower SYMCOD rate channel. The terminal again sees excess SINR and this problem iteratively repeats itself.
Accordingly, it is important for a terminal to account for the difference in SINR of the channels (in addition to SYMCOD) before deciding to change channels.
The conventional method of addressing this problem relies on manual use of a spectrum analyzer at the gateway. Under this conventional approach, the spectrum analyzer is used to measure the spectrum of each IFC manually, and this is followed by manually post processing each of the measured spectrums in attempt to determine the margins for every IFC for every group of inroutes. The determined margins are entered into a calibration table that may be made available to the terminals.
The conventional spectrum analyzer method has many drawbacks. First, it is a cumbersome and manual-labor intensive process, particularly when measuring all satellite bands, spot beams, and polarizations. Further, the method is limited to measuring the spectrum of a small subset of terminals. Additionally, the conventional method requires that all the user terminals operating in a band be barred from transmission to have an accurate representation of the noise and interference present in the band that is being calibrated, thereby preventing the user terminals from accessing the internet. Further, still, the process is ill-suited for the frequent system recalibrations desired due to 1) the dynamic nature of interference caused by sources other than thermal noise, and 2) changes in the satellite system configuration.